Runnable simulator field guide
CTU counter curriculum scenario: test checklist
Direct answer
The learner can distinguish a level from a rising event, predict accumulator and done state, test reset priority and explain scan and restart assumptions.
Written for pLC beginners learning how a CTU instruction turns discrete events into a retained count and done condition.
Scenario setup
Counted event, input pulse shape, instruction instance, rising-edge rule, accumulator, preset, done state, reset condition, initial value, data limit and retention assumption.
I/O and state map
Physical or simulated event through input sampling and edge detection to CTU instance state, done condition, downstream action and displayed count.
Normal run
Each deliberate false-to-true event adds exactly one count and the done state changes at the declared preset.
Boundary run
Held input, contact bounce, rapid pulses, simultaneous count and reset, zero preset, maximum accumulator, overflow, restart and retained state.
Injected fault
An event, input, scan, edge, instance, accumulator, preset, reset, data-limit or restart mismatch.
Target check
The same cases recreated using the exact target instruction and controller documentation.
What makes a CTU counter increment?
A count-up instruction normally responds to a defined false-to-true event at its count input, but exact platform behavior must be verified.
Why does a PLC counter miss or add events?
Pulse width, scan or task timing, contact bounce, input filtering, edge handling and duplicate instruction execution can change the observed count.